When you’ve had a truly terrible night of sleep, you know it. But if you’re chronically sleep deprived, it may not be as obvious. Certain behaviors that you might chalk up to a funk or hormone swings could, in fact, be signs that you’re not getting enough zzzs.

“We’re kind of a nation of walking zombies,” says Dr. James Maas, author of Power Sleep and Sleep For Success! Everything You Must Know About Sleep But Are Too Tired To Ask. “Most people need one hour more than what they’re getting.”

Chances are, you’re overestimating how much sleep you’re getting by 30-60 minutes. And you may not even realize you’re not as alert as you could be because you’ve gotten used to a lower energy level.

Check out this list of signs you’re not getting enough sleep, and if any of them sound familiar, brush up on your better-sleep basics.

You’re irritable or moody.“People are generally a little more irritable when they’re sleep deprived,” says Dr. Bradley Vaughn, a professor of neurology and the chief ofthe Division of Sleep and Epilepsy at the University of North Carolina School of Medicine. “They’re less forgiving. They’re a little more willing to take an opposing side” in a discussion.

You depend heavily on your alarm clock. If your body is getting enough rest, it will know when it’s time to wake up. People who can’t get out of bed for the day without an alarm clock are sleep-deprived, says Maas. To help get yourself into a healthy sleep-wake cycle, you should rise at about the same time each day. If you sleep later on the weekend, all that does is confuse your body’s internal clock. It’s like getting jet lag every week, says Vaughn.

You can’t concentrate. If you have to read something several times to really understand it, your concentration may have been hampered by a lack of zzzz’s. A “power nap” of 10-15 minutes may help, says Maas, who coined the term, but don’t sleep any longer or you’ll wake up feeling groggy.

You’re inexplicably hungry. A 2008 study in the Journal of Sleep Research found that just one night of only 4-and-a-half hours of sleep increased hunger, due to a spike in the hormone ghrelin. Other studies, including one presented at the 2010 conference of the Endocrine Society, have found that ghrelin specifically causes cravings for higher-calorie foods.

You fall asleep very quickly. It may sound counter-intuitive, but a sure sign of sleep deprivation is falling asleep within five minutes of hitting the pillow. “The well-rested person takes 15 to 20 minutes to go to sleep,” says Maas.